Stimulation of R cells with IL5 induces the appearance of a phenotypically novel B cell population which expresses high density of CD44 and low densities of B22O (CD45) and Ia. CD44 expressed by these cells mediates binding to the extracellular matrix material hyaluronic acid (HA), indicating a potential role for CD44 in trafficking of activated B cells in vivo. CD44 expressed on IL5-stimulated B cells migrates with a lower molecular weight than CD44 expressed by control B cells, reflecting differential glycosylation. No differences in CD44 mRNA isoform were apparent by PCR analysis. The B cell stimuli LPS and anti-5 do not induce CD44-dependent HA-binding activity. However, LPS-activated B cells demonstrate CD44-dependent HA binding rapidly after exposure to a unique CD44-specific mAb, suggesting that distinct functional states of the GD44 molecule exist reflecting differences in conformation or cytoskeletal association. mAb was generated by immunizing rats with activated mouse B cells. One of these mAb (GL7) reacts with a subpopulation of activated B cells, as well as with activated T cells. GL7 precipitates a previously undescribed 29- 31 KDa molecule from activated B cells. Another mAb (GL1) reacts with activated B cells. GL1 inhibits responses of CD4+ T cells to activated B cells, suggesting that the target of GL1 may represent a costimulatory molecule for T cell activation. To establish a system for the study of Th cell-R cell interaction at a single cell level, responses were generated using Ig transgenic B cells and cloned Th cells. Highly efficient hapten-specific responses were generated. The study of sera from these transgenic mice indicated that transgene-associated idiotype was expressed in association with endogenous Ig molecules.